RPM with given distance

A space station is in the shape of a hollow ring, 450 m in diameter. At how many revolutions per minute should it rotate in order to simulate Earth's gravity-that is, so that the normal force on an astronaut at the outer edge would be the astronaut's weight on Earth?

All I got so far was finding the circumference=pid=1413 meters. I'm stuck, I don't know what to do next. Help please!